Predictors of positive blood cultures in critically ill patients: a retrospective evaluation

To identify predictors of bacteremia in critically ill patients, to evaluate the impact of blood cultures on the outcome, and to define conditions for breakthrough bacteremia despite concurrent antibiotic treatment.

A descriptive retrospective study was performed over a two-year period (2007-2008) in the medico-surgical Intensive Care Unit (ICU) of the San Giovanni Hospital in Bellinzona, Switzerland.

Forty-five out of 231 patients (19.5%) had positive blood cultures. Predictors of positive blood cultures were elevated procalcitonin levels (>2 µg/L, P<0.001), higher severity scores (Simplified Acute Physiology Score II>43, P=0.014; Sequential Organ Failure Assessment >4.0, P<0.001), and liver failure (P=0.028). Patients with bacteremia had longer hospital stays (31 vs 21 days, P=0.058), but their mortality was not different from patients without bacteremia. Fever (t>38.5°C) only showed a trend toward a higher rate of blood culture positivity (P=0.053). The rate of positive blood cultures was not affected by concurrent antibiotic therapy.

The prediction of positive blood culture results still remains a very difficult task. In our analysis, blood cultures were positive in 20% of ICU patients whose blood was cultured, and positive findings increased with elevated procalcitonin levels, liver failure, and higher severity scores. Blood cultures drawn >4 days after the start of antibiotic therapy and >5 days after surgery could detect pathogens responsible for a new infection complication.